Missing constant causes admin page not to load styles

[HFigarella]

BUG

In the last alpha versions for the past week including 5.8-alpha-51047

...\wp-includes\theme.php is triggering this error reported by PHP 8.0.6

PHP Fatal error: Uncaught Error: Undefined constant "WP_CONTENT_DIR" in \wp-includes\theme.php:593

Consequently, none of the css files are loaded in the admin page

My solution

Problem is solved by defining the missing constant before line 593 ($theme_root = WP_CONTENT_DIR . $theme_root)

<?php
function get_theme_root( $stylesheet_or_template = '' ) {
        global $wp_theme_directories;

        // Added by Horacio. Solves error of missing constant when upgrading to WP 5.8 PHP Fatal error:  Uncaught Error: Undefined constant "WP_CONTENT_DIR" in \wp-includes\theme.php:599
        if ( ! defined( 'WP_CONTENT_DIR' ) ) define( 'WP_CONTENT_DIR', ABSPATH . 'wp-content' );

        $theme_root = '';

        if ( $stylesheet_or_template ) {
                $theme_root = get_raw_theme_root( $stylesheet_or_template );
                if ( $theme_root ) {
                        // Always prepend WP_CONTENT_DIR unless the root currently registered as a theme directory.
                        // This gives relative theme roots the benefit of the doubt when things go haywire.
                        if ( ! in_array( $theme_root, (array) $wp_theme_directories, true ) ) {
                                $theme_root = WP_CONTENT_DIR . $theme_root;
                        }
                }
        }

        if ( ! $theme_root ) {
                $theme_root = WP_CONTENT_DIR . '/themes';
        }

Alternative solution is to install WordPress 5.7.2 or any 5.8 alpha version but the ones from last week.

[SergeyBiryukov]

Hi there, welcome to WordPress Trac!

Thanks for the report, we're already tracking this issue in #53175, starting with comment:34:ticket:53175.